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2cos^3(x)+cos^2(x)-2cos(x)-1=0

Solución

2 cos^{3} (x) + cos^{2} (x) - 2 cos (x) - 1 = 0

Solución

x = \frac{2 π}{3} + 2 πn, x = \frac{4 π}{3} + 2 πn, x = π + 2 πn, x = 2 πn

Grados

x = 12 0^{\circ} + 36 0^{\circ} n, x = 24 0^{\circ} + 36 0^{\circ} n, x = 18 0^{\circ} + 36 0^{\circ} n, x = 0^{\circ} + 36 0^{\circ} n

Pasos de solución

2 cos^{3} (x) + cos^{2} (x) - 2 cos (x) - 1 = 0

Usando el método de sustitución

2 cos^{3} (x) + cos^{2} (x) - 2 cos (x) - 1 = 0

Sea:

cos (x) = u

2 u^{3} + u^{2} - 2 u - 1 = 0

2 u^{3} + u^{2} - 2 u - 1 = 0 : u = - \frac{1}{2}, u = - 1, u = 1

2 u^{3} + u^{2} - 2 u - 1 = 0

Factorizar

2 u^{3} + u^{2} - 2 u - 1 : (2 u + 1) (u + 1) (u - 1)

2 u^{3} + u^{2} - 2 u - 1

= (2 u^{3} + u^{2}) + (- 2 u - 1)

Factorizar

- 1

- 2 u - 1

- (2 u + 1)

- 2 u - 1

Factorizar el termino común

- 1

= - (2 u + 1)

Factorizar

u^{2}

2 u^{3} + u^{2}

u^{2} (2 u + 1)

2 u^{3} + u^{2}

Aplicar las leyes de los exponentes:

a^{b + c} = a^{b} a^{c}

u^{3} = u u^{2}

= 2 u u^{2} + u^{2}

Factorizar el termino común

u^{2}

= u^{2} (2 u + 1)

= - (2 u + 1) + u^{2} (2 u + 1)

Factorizar el termino común

2 u + 1

= (2 u + 1) (u^{2} - 1)

Factorizar

u^{2} - 1 : (u + 1) (u - 1)

u^{2} - 1

Reescribir

1

como

1^{2}

= u^{2} - 1^{2}

Aplicar la siguiente regla para binomios al cuadrado:

x^{2} - y^{2} = (x + y) (x - y)

u^{2} - 1^{2} = (u + 1) (u - 1)

= (u + 1) (u - 1)

= (2 u + 1) (u + 1) (u - 1)

(2 u + 1) (u + 1) (u - 1) = 0

Utilizando el teorema de factor cero: si

ab = 0

entonces

a = 0

b = 0

2 u + 1 = 0 or u + 1 = 0 or u - 1 = 0

Resolver

2 u + 1 = 0 : u = - \frac{1}{2}

2 u + 1 = 0

Mover

1

al lado derecho

2 u + 1 = 0

Restar

1

de ambos lados

2 u + 1 - 1 = 0 - 1

Simplificar

2 u = - 1

2 u = - 1

Dividir ambos lados entre

2

2 u = - 1

Dividir ambos lados entre

2

\frac{2 u}{2} = \frac{- 1}{2}

Simplificar

u = - \frac{1}{2}

u = - \frac{1}{2}

Resolver

u + 1 = 0 : u = - 1

u + 1 = 0

Mover

1

al lado derecho

u + 1 = 0

Restar

1

de ambos lados

u + 1 - 1 = 0 - 1

Simplificar

u = - 1

u = - 1

Resolver

u - 1 = 0 : u = 1

u - 1 = 0

Mover

1

al lado derecho

u - 1 = 0

Sumar

1

a ambos lados

u - 1 + 1 = 0 + 1

Simplificar

u = 1

u = 1

Las soluciones son

u = - \frac{1}{2}, u = - 1, u = 1

Sustituir en la ecuación

u = cos (x)

cos (x) = - \frac{1}{2}, cos (x) = - 1, cos (x) = 1

cos (x) = - \frac{1}{2}, cos (x) = - 1, cos (x) = 1

cos (x) = - \frac{1}{2} : x = \frac{2 π}{3} + 2 πn, x = \frac{4 π}{3} + 2 πn

cos (x) = - \frac{1}{2}

Soluciones generales para

cos (x) = - \frac{1}{2}

cos (x)

tabla de valores periódicos con

2 πn

intervalos:

x 0 \frac{π}{6} \frac{π}{4} \frac{π}{3} \frac{π}{2} \frac{2 π}{3} \frac{3 π}{4} \frac{5 π}{6} cos (x) 1 \frac{3}{2} \frac{2}{2} \frac{1}{2} 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} x π \frac{7 π}{6} \frac{5 π}{4} \frac{4 π}{3} \frac{3 π}{2} \frac{5 π}{3} \frac{7 π}{4} \frac{11 π}{6} cos (x) - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2} 0 \frac{1}{2} \frac{2}{2} \frac{3}{2}

x = \frac{2 π}{3} + 2 πn, x = \frac{4 π}{3} + 2 πn

x = \frac{2 π}{3} + 2 πn, x = \frac{4 π}{3} + 2 πn

cos (x) = - 1 : x = π + 2 πn

cos (x) = - 1

Soluciones generales para

cos (x) = - 1

cos (x)

tabla de valores periódicos con

2 πn

intervalos:

x 0 \frac{π}{6} \frac{π}{4} \frac{π}{3} \frac{π}{2} \frac{2 π}{3} \frac{3 π}{4} \frac{5 π}{6} cos (x) 1 \frac{3}{2} \frac{2}{2} \frac{1}{2} 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} x π \frac{7 π}{6} \frac{5 π}{4} \frac{4 π}{3} \frac{3 π}{2} \frac{5 π}{3} \frac{7 π}{4} \frac{11 π}{6} cos (x) - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2} 0 \frac{1}{2} \frac{2}{2} \frac{3}{2}

x = π + 2 πn

x = π + 2 πn

cos (x) = 1 : x = 2 πn

cos (x) = 1

Soluciones generales para

cos (x) = 1

cos (x)

tabla de valores periódicos con

2 πn

intervalos:

x 0 \frac{π}{6} \frac{π}{4} \frac{π}{3} \frac{π}{2} \frac{2 π}{3} \frac{3 π}{4} \frac{5 π}{6} cos (x) 1 \frac{3}{2} \frac{2}{2} \frac{1}{2} 0 - \frac{1}{2} - \frac{2}{2} - \frac{3}{2} x π \frac{7 π}{6} \frac{5 π}{4} \frac{4 π}{3} \frac{3 π}{2} \frac{5 π}{3} \frac{7 π}{4} \frac{11 π}{6} cos (x) - 1 - \frac{3}{2} - \frac{2}{2} - \frac{1}{2} 0 \frac{1}{2} \frac{2}{2} \frac{3}{2}

x = 0 + 2 πn

x = 0 + 2 πn

Resolver

x = 0 + 2 πn : x = 2 πn

x = 0 + 2 πn

0 + 2 πn = 2 πn

x = 2 πn

x = 2 πn

Combinar toda las soluciones

x = \frac{2 π}{3} + 2 πn, x = \frac{4 π}{3} + 2 πn, x = π + 2 πn, x = 2 πn

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